Observational Constraints on the Oxidation of NOx in the Upper

Article pubs.acs.org/JPCA

Observational Constraints on the Oxidation of NOx in the Upper Troposphere Benjamin. A. Nault,† Charity Garland,‡ Paul J. Wooldridge,‡ William H. Brune,§ Pedro Campuzano-Jost,∥ John D. Crounse,⊥ Douglas A. Day,∥ Jack Dibb,∇ Samuel R. ̂ Hall,¶ L. Gregorŷ Huey,# José L. Jimenez,∥ ○ Xiaoxi Liu,# Jingqiu Mao, Tomas Mikoviny,◆ Jeff Peischl,$ Ilana B. Pollack,$,% Xinrong Ren,+ ̂ Thomas B. Ryerson,$ Eric Scheuer,∇ Kirk Ullmann,¶ Paul O. Wennberg,^ Armin Wisthaler, = Li Zhang,§ and Ronald C. Cohen*,†,‡ †

Department of Earth and Planetary Science and ‡Department of Chemistry, University of California, Berkeley, California 94709, United States § Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ Cooperative Institute for Research in the Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ⊥ Division of Geological and Planetary Sciences and ^Division of Engineering and Applied Science and Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, United States ∇ Earth Systems Research Center, Institute for the Study of Earth Oceans and Space, University of New Hampshire, Durham, New Hampshire 03824, United States ¶ Atmospheric Chemistry Division, National Center for Atmospheric Research (NCAR), Boulder, Colorado 80307, United States # School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ○ Geophyiscal Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey 08540, United States ◆ Oak Ridge Associated Universities, Oak Ridge, Tennessee 37831, United States $̂ Chemical Sciences Division, Earth System Research Lab, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, United States + Air Resources Laboratory, National Oceanic and Atmospheric Administration, College Park, Maryland 20740, United States = Institute of Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria S Supporting Information *

ABSTRACT: NOx (NOx ≡ NO + NO2) regulates O3 and HOx (HOx ≡ OH + HO2) concentrations in the upper troposphere. In the laboratory, it is difficult to measure rates and branching ratios of the chemical reactions affecting NOx at the low temperatures and pressures characteristic of the upper troposphere, making direct measurements in the atmosphere especially useful. We report quasi-Lagrangian observations of the chemical evolution of an air parcel following a lightning event that results in high NOx concentrations. These quasi-Lagrangian measurements obtained during the Deep Convective Clouds and Chemistry experiment are used to characterize the daytime rates for conversion of NOx to different peroxy nitrates, the sum of alkyl and multifunctional nitrates, and HNO3. We infer the following production rate constants [in (cm3/molecule)/s] at 225 K and 230 hPa: 7.2(±5.7) × 10−12 (CH3O2NO2), 5.1(±3.1) × 10−13 (HO2NO2), 1.3(±0.8) × 10−11 (PAN), 7.3(±3.4) × 10−12 (PPN), and 6.2(±2.9) × 10−12 (HNO3). The HNO3 and HO2NO2 rates are ∼30−50% lower than currently recommended whereas the other rates are consistent with current recommendations to within ±30%. The analysis indicates that HNO3 production from the HO2 and NO reaction (if any) must be accompanied by a slower rate for the reaction of OH with NO2, keeping the total combined rate for the two processes at the rate reported for HNO3 production above.

1. INTRODUCTION Special Issue: James G. Anderson Festschrift

In the upper troposphere, where O3 is an important greenhouse gas,1 NOx (NOx ≡ NO + NO2) regulates O3 production. NOx concentrations in this region of the atmosphere are affected by transport including import from the stratosphere, convection, © XXXX American Chemical Society

Received: August 11, 2015 Revised: November 16, 2015

A

DOI: 10.1021/acs.jpca.5b07824 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A and large-scale descent, emissions from aircraft, in situ production by lightning, and photochemical reactions that convert NOx to higher oxides, or NOz (Figure 1).2−5 The

peroxy acetyl nitrate (PAN) and proxy propionyl nitrate (PPN) at temperatures less than 240 K. The main sink of NOx, formation of HNO3, has been subject of the most laboratory experiments. Total loss of OH in the reaction of OH with NO2 is known to represent production of HNO3 (R1a) and peroxynitrous acid (HOONO, reaction R1b).9 The production of HOONO has been studied at room temperature,9 where the branching is 87% to HNO3 and 13% to HOONO. Significant extrapolations are involved at the temperatures and pressures of the upper troposphere. Estimates of the branching are 75% for HNO3 and 25% for HOONO at T = 225 K and P = 230 hPa.10 In addition to reaction R1a, some laboratory studies11−13 have measured the yield of HNO3 from the reaction of HO2 and NO (R2b) and suggest that this minor branch has nonzero yield of less than 1%. Because the overall rate of HO2 and NO is on the order of 10−100 times faster than the OH and NO2 reaction in the atmosphere, this low yield, if correct, represents an important sink of NOx and source of HNO3. Laboratory measurements of yields in this range are extremely difficult as secondary reactions (e.g., reaction R2a then reaction R1a) in the gas phase or on the walls of the reaction vessel can be significant interferences. Other sinks of NOx are also important in the upper troposphere. For example, pernitric acid (HO2NO2) and methyl peroxy nitrate (CH3O2NO2) are important short-lived NOx reservoirsless than 24 h through thermal decomposition (R5), photolysis (R6), or reaction of HO2NO2 with OH (R7). For the conditions of the upper troposphere, several studies of the rate of HO2 with NO2 are available,14−17 and one study for the rate of CH3O2 with NO218 is available. Bacak et al.14 report that the HO2NO2 production rate is almost 50% slower than currently recommended10 and observed in prior experiments.15−17

Figure 1. Daytime NOx fate in the upper troposphere. Reactions or channels that are not NOx sinks (e.g., reaction R2a) are not included. Double arrows indicate important thermal decomposition reactions that occur in the upper troposphere. Arrows and compounds in red (black) indicate reactions and products with NO (NO2), and the blue arrow and reactants (products) in the bracket correspond to all the products. The lifetime for conversion back to NOx in the upper troposphere by photolysis and reactions with OH is days for all species except HO2NO2 and HOONO. For these two species, the lifetime is hours in the upper troposphere.

higher oxides include HNO3 (reaction R1a and R2b), alkyl and multifunctional nitrates (reaction R3b), and acyl and nonacyl peroxy nitrates (reactions R4 and R5). k1a

NO2 + OH + M → HNO3 + M k1b

NO2 + OH + M → HOONO + M k −1b

HOONO + M ⎯⎯⎯→ NO2 + OH + M k 2a

NO + HO2 → NO2 + OH

(R1b)

k6a

(R−1b)

RO2 NO2 (HO2 NO2 ) + hν → NO2 + RO2•(HO2•) (R6a)

(R2a)

k 2b

NO + HO2 → HNO3

k6b

RO2 NO2 (HO2 NO2 ) + hν → NO3 + RO•(HO•)

(R2b)

k 3a

NO + RO2• → NO2 + RO• • k 3b

NO + RO2 → RONO2

(R3a)

k7

HO2 NO2 + OH → NO2 + O2 + H 2O

(R3b)

k4

•

(R1a)

NO2 + R(O)O2 + M → R(O)O2 NO2 + M k −4

R(O)O2 NO2 + M ⎯→ ⎯ NO2 + R(O)O2• + M

(R6b)

(R7)

Any increases in the upper tropospheric NOx lifetime will lead to increases in O3 concentration.14,19 This leads to higher O3 radiative impacts. Recently, Romps et al.20 described an analysis of lightning that predicts its increase in a warmer climate. The additional NOx associated with lightning will result in higher O3 than today in the upper troposphere and increases in its radiative forcing; thus, it is important to understand NOx lifetime in the upper troposphere. To investigate the rates of NOx oxidation, we analyze a quasiLagrangian experiment that observed the temporal evolution of NO, NO2, CH3O2NO2, HO2NO2, PAN, PPN, and the sum of alkyl and multifunctional nitrates, HNO3, and submicron aerosol nitrate downwind of a lightning storm on June 21st, 2012. The observations were obtained during the Deep Convective Clouds and Chemistry (DC3) experiment.21 To assess the impacts of the rate coefficients we derive on the chemistry of NOx in the upper troposphere, we use a global chemical transport model to compare O3, OH, HO2, and NOx for standard chemistry and the modified set of rate constants.

(R4) (R−4)

k5

NO2 + RO2•(HO2•) + M → RO2 NO2 (HO2 NO2 ) + M (R5) k −5

RO2 NO2 (HO2 NO2 ) + M ⎯→ ⎯ NO2 + RO2•(HO2•) + M (R−5)

It has been typically assumed (with support from calculations) that production of HNO3 strongly regulates the upper tropospheric NOx lifetime with other NOz playing a minor role.6−8 However, there are relatively few laboratory studies constraining the rate constants for the production of NOz under the conditions of the upper troposphere (T less than 250 K and P less than 400 hPa). For example, there are no laboratory observations of the rate constants for production of B


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The Journal of Physical Chemistry A

2. INSTRUMENTATION The observations described here were made aboard the NASA DC-8 during the DC3 experiment (May−June 2012).21 All data used in this study can be accessed through the NASA public archives for this mission.22 We use the Version 5 60 s merged files. NO2 was measured by the UC Berkeley thermal-dissociation laser-induced fluorescence (TD-LIF) instrument.23 Briefly, a tunable dye laser pumped at 7 kHz by a Q-switched, frequency doubled Nd:YAG laser, produces ∼20 ns pulses at 585 nm with a bandwidth of 0.06 cm−1. The dye laser is tuned onto (9 s) and off (3 s) an isolated rovibronic feature of the jet-cooled24 NO2 at 585 nm. The difference between the two signals is directly proportional to the NO2 mixing ratios. At the pressures of this flight (230 hPa), the ratio of the resonant peak to the nonresonant background fluorescence is 10 to 1. The laser light is focused through two multipass White cells. The red-shifted fluorescence from the excited NO2 molecules is detected using a red-sensitive photomultiplier tube (Hamamatsu H7421-50). Scattered light is removed using time-gated detection and dielectric band-pass filters that block light at wavelengths shorter than 700 nm. The instrument is calibrated at least every hour during a level flight leg or after a significant change in altitude using a 4.67(±0.26) ppmv NO2 standard (Praxair) diluted to ∼2−8 ppbv in zero air. The accuracy and stability of the reference is compared against a library of other NO2 standards in our laboratory on a regular basis. Fluorescence quenching by water vapor is accounted for23 using the diode laser hygrometer (DLH) measurements of H2O.25 Partial thermal decomposition of CH3O2NO2 in the NO2 sample26 is corrected by concurrent measurements of CH3O2NO2 using thermal-dissociation with laser-induced fluorescence detection of the NO2 enhancement.27 During this flight, the maximum correction to the NO2 measurements due to thermal decomposition of CH3O2NO2 is 10% with a median correction of 7% for the entire flight. The correction is described in Nault et al.27 The measurements of CH3O2NO2, sum of all peroxy nitrates (ΣPNs = PAN + PPN + HO2NO2 + CH3O2NO2), and sum of alkyl and multifunctional nitrates in the gas and particle phase (ΣANs(g+p) = all molecules of the chemical form RONO2) have been described elsewhere27−30 and the specific configuration during DC3 by Nault et al.27 Briefly, the CH3O2NO2, ΣPNs, and ΣANs(g+p) are detected by thermal conversion to NO2 in heated quartz tubes (60, 200, and 400 °C, respectively) and detected by laser-induced fluorescence of the NO2 fragment. The concentrations of CH3O2NO2 and ΣPNs are calculated from the difference in the signal between the ambient NO2 measurements and the 60 and 200 °C channel, respectively. Similarly, the concentrations of ΣANs(g+p) are calculated from the difference in the signal between the 200 and 400 °C channel. The temperature selected for the thermal decomposition (60 °C) of CH3O2NO2 was selected to minimize the amount of HO2NO2 that thermally decomposed. At that temperature and with a residence time of 0.08 s, we calculated that ∼10% of HO2NO2 thermally decomposes.27 The CH3O2NO2 measurements are corrected using HO2NO2 observed by chemical ionization-mass spectrometry (CIMS)31 as described by Nault et al.27 The maximum correction is 10% with a median correction of 5% for the entire flight. Our estimates indicate

that no other peroxy nitrates have important effects on the CH3O2NO2 and ΣPNs measurements.27 In addition to the NO2, CH3O2NO2, ΣPNs, and ΣANs(g+p) measurements, we use the measurements of NO and O3 by chemiluminescence,32 OH and HO2 by laser-induced fluorescence,33 gas-phase nitric acid (HNO3,g), CH3OOH, CH3C(O)OOH, and hydroxyacetone by CIMS,34,35 particle-phase inorganic nitrate (NO3−) and particulate organic nitrate (ΣANs(p)) by aerosol mass spectrometry (AMS),36,37 the sum of gas- and particle-phase nitric acid (HNO3(g+p)) by mist chamber-ion chromatography (IC),38 hydrocarbons (i.e., ethane, propane, ethene, n-butane) by whole air sampling with gas-chromatography analsysis,39 PAN, PPN, and HO2NO2 by chemical ionization-mass spectrometry,31,40 methyl vinyl ketone (MVK), methacrolein (MACR), acetone, acetaldehyde, isoprene, benzene, toluene, C10 terpenes (i.e., α-pinene), and C8 aromatics (i.e., m-xylene) by proton transfer reaction-mass spectrometry,41 and CH4 by tunable diode laser absorption spectroscopy25,42 (Table 1). Here, HNO3(g+p) is taken as the Table 1. Measurements Used in This Analysis to Calculate Rate Constants, Peroxy Radical Concentrations, Average α, and ΣANs Production Rate instrument chemiluminescencea thermal-dissociation laser-induced fluorescenceb laser-induced fluorescencec chemical ionization-mass spectrometryd aerosol mass spectrometrye mist chamber-ion chromatographyf chemical ionization-mass spectrometryg spectral radiometryh whole air sampleri proton transfer reaction mass spectrometryj tunable diode laser absorption spectroscopyk

species NO, O3 NO2, CH3O2NO2, ΣPNs, ΣANs(g+p) OH, HO2 HNO3,g, CH3OOH, CH3C(O)OOH, hydroxyacetone NO3−(p), ΣANs(p) HNO3(g+p) PAN, PPN, HO2NO2 photolysis rates hydrocarbons acetaldehyde, acetone, benzene, C8 aromatics, isoprene, monoterpenes, MVK + MACR, toluene CH4

a

Reference 32. bReferences 23 and 27−30. cReference 33. dReferences 34 and 35. eReferences 36 and 37. fReference 38. gReferences 31 and 40. hReference 63. iReference 39. jReference 41. kReferences 25 and 42.

average of the CIMS and IC measurements. An intercomparison and treatment of the HO2NO2, peroxy nitrates, and HNO3(g+p) measurements are described in the Supporting Information. Also, an intercomparison of photostationary steady state NO2 versus measured NO2 is described in the Supporting Information.

3. RESULTS On 21 June 2012, the NASA DC-8 sampled the photochemistry of a decaying mesoscale convective system over the central United States (Figure 2) at a near constant temperature and pressure of 225 K and 230 hPa (∼7.3 × 1018 molecules/ cm3). The decaying mesoscale convective system and flight are described in Barth et al.21 Briefly, the DC-8 flew 5 legs spaced C


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Figure 3. Time series of (a) NO and NO2, (b) ΣPNsi, (c) ΣANs(g+p), and (d) HNO3(g+p), where ΣPNsi = CH3O2NO2 + HO2NO2 + PAN + PPN and HNO3(g+p) is the average of the IC and CIMS data. Note, NO2 is blue and the left y-axis and NO is green and the right y-axis in (a). Measurements not shown between 14:30 and 15:10 UTC are when the DC-8 sampled air outside the Lagragian parcel. The vertical red lines correspond to the beginning of Legs 1, 2, and 3 in Figure 2.

Figure 2. Portions of the NASA DC-8 flight path (blue) used to calculate all the rate constants. The wind direction and relative speed observed on the DC-8 for the 21 June 2012 flight is shown as red arrows. The edge of the cloud that marks time equals 0 for the Lagrangian observations is shown as the green dashed-dot line. Legs 1, 2, and 3 corresponds to the leftmost, center, and rightmost flight tracks. Red arrow and black horizontal line, at bottom of figure, correspond to 25 m/s wind speed and 88 km distance, respectively.

The ΣANs(g+p) concentration grew 45(±20) pptv, from an initial 105 pptv, or 4.1(±1.8) × 104 (molecules/cm3)/s, whereas ΣANs(p) increased by 7(±2) pptv, from an initial 5 pptv. Finally, HNO3(g+p) increased by 40(±5) pptv, from an initial 38 pptv, or 3.6(±0.5) × 104 (molecules/cm3)/s. The loss rate of NOx is, within the measurement uncertainty, identical to the production rate of the higher oxide species (2.3(±0.3) × 105 (molecules/cm3)/s), indicating the observations of the decay of NOx are consistent with the formation of NOz. Analysis of observations of ethene, ethyne, and toluene confirm the air mass is evolving in isolation with little mixing. These three hydrocarbons are selected because their only known source in the upper troposphere is convection.43,44 OH was constant to within 25% in this segment of the flight at 9.5 × 106 molecules/cm3. The OH concentration stayed constant during this time because its sources (e.g., CH2O) and sinks (e.g., NOx) decayed at similar rates. Using the measured decay of etheneinitial median mixing ratio of 43 pptv and final median mixing ratio of 14 pptvand this OH concentration, we find a value of 1.0(±0.4) × 10−11 (cm3/molecule)/s (Figure 4) for the rate constant for the reaction of ethene with OH. The 2σ uncertainty of 40% is determined by adding the uncertainty of the slope (±7%) and the uncertainty of the OH measurement (1σ ≈ 16%) in quadrature. This is nearly identical −12 to the recommended rate constant of 8.9+2.1 (cm3/ −1.9 × 10 10 molecule)/s at 225 K and 230 hPa. Doing a similar analysis with the decay of ethyne (initial and final median values of 98 to 89 pptv, respectively) and toluene (initial and final median values of 25 pptv to 14 pptv, respectively), we find values of 6.3(±3.5) × 10−13 and 1.1(±0.4) × 10−11 (cm3/molecule)/s (Figure S5), respectively. These ethyne and toluene rate constants are also nearly identical to the recommended rate −13 −12 constants of 7.0+1.1 and 8.8+8.2 (cm3/ −1.0 × 10 −4.3 × 10 10,45 molecule)/s, respectively. The 2σ is calculated the same as for the ethene rate constant. In addition to confirming the air parcel is evolving in isolation, the ability to calculate these three rate constants within the uncertainty of the recommended values support the combined assumptions made about the evolution of the air parcel as a quasi-Lagrangian air parcel for 8100 s. If we assume the uncertainty in the slopes of the calculations is largely driven by uncertainty in the elapsed time,

at varied distances downwind of the convection (full flight Figure S3). Portions of the last three legs that we characterize as sampling the outflow in a quasi-Lagrangian sense are shown in Figure 2, along with our estimate of the location of the cloud outflow marked as the green dashed-dot line. We define time zero as the first point in our sampling at 13:35 UTC. We estimate this was less than 10 min after the air left the cloud. The observations used in our analysis are those that are in the portions of the flight track that were approximately perpendicular to the flow. As the outflow was a line source (green line in Figure 2), all points sampled can be thought of as having a common chemical origin (entire green line) and time evolution from the source of the outflow. The specific data we use are measurements initialized at 13:35 UTC (7:35 local time) and continuing to 8100 s after the initial time (15:50 UTC). Using the distance between the legs (∼50 and 100 km) and the wind speed (∼25 m/s), we calculate that it takes the air ∼2000 s to travel from a point in leg 1 to 2 and 4000 s from a point in leg 2 to 3, respectively. On the basis of this calculation, the observation time (relative to 13:35 UTC) and the transit time from that point are approximately equal, and we define the observation time difference as the time that the air parcel has evolved in the subsequent analysis. Initializing the observations at the point observed at 13:35 UTC, we observed a decay in NOx and production of CH3O2NO2, HO2NO2, PAN, PPN, ΣPNsi, ΣANs(g+p), and HNO3(g+p) (Figure 3 and S4). NO and NO2 decayed by 190(±70) and 90(±20) pptv from an initial median mixing ratio of 670 and 270 pptv, respectively. The uncertainty in the decay represents the interquartile of the change. This corresponds to a total loss rate of 2.6(±0.6) × 10 5 (molecules/cm3)/s, if we assume chemical loss of NOx is constant and linear over the 8100 s between the initial and final observation. Over the same interval, the production of the four peroxy nitrates was 140(±25) (CH 3 O 2 NO 2 ), 20(±5) (HO2NO2), 15(±5) (PAN), and 5(±1) (PPN) pptv. The initial median mixing ratios for the four peroxy nitrates was 0 (CH3O2NO2), 50 (HO2NO2), 210 (PAN), and 20 (PPN) pptv. D


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incorporated in the uncertainty of the slope. We find that the rate constant is 6.2(±2.9) × 10−12 (cm3/molecule)/s, with the 2σ uncertainty defined as for ethene. This rate constant (T = 225 K and P = 230 hPa) is slower than recommended by Atkinson et al.46 and Sander et al.10 and nearly identical to the recommendation of Henderson et al.,47 who derived a slower rate on the basis of observations of NO x and HNO 3 concentrations in the upper troposphere during the Intercontinental Chemical Transport Experiment−North America (Figure 6). Though the rate constant we calculated is lower

Figure 4. First-order loss rate of ethene. The solid red line is the slope (−9.6 × 10−5 s−1), and the dashed-dot red line is the 2σ uncertainty (±1.4 × 10−5 s−1).

then the uncertainty in time is 10−40%. However, other uncertainties likely contribute, indicating a smaller range for the uncertainty in reaction time is more likely. In conclusion, the analysis of the hydrocarbon decay confirms that the initial observations (t = 0) occurred at 13:35 UTC and confirms our time coordinates as time in seconds after 13:35 UTC.

4. ANALYSIS 4.1. Nitric Acid Production. We apply the same assumptions to analysis of the rate of oxidation of NOx, starting with an analysis of HNO3(g+p) production. The HNO3(g+p) production rate, dHNO3(p+g)/dt, is calculated by subtracting the initial HNO3(g+p) concentrations at time 0, which is 2.8 × 108 molecules/cm3 at 13:35 UTC, from HNO3(g+p) concentrations at time t. We use small time increments to account for other processes affecting the NO2 available for reactions (reactions R4 and R5 and photolysis). The measured dHNO3(g+p)/dt is plotted versus the corresponding NO2 concentration at that time, t (Figure 5). Similar to the hydrocarbon rate constant calculation (section 3), we assume any uncertainty in the time after the initial observation is

Figure 6. Comparison of k1a rate constant at T = 225 K and M = 7.3 × 1018 molecules/cm3 from Sander et al.,10 Atkinson et al.,46 Henderson et al.,47 and this study (red). The range shows the 2σ uncertainty of the product of the rate constant and branching ratio for HNO3 formation.

than Sander et al. by 30%, the value we calculated overlaps with the lower bound of the 2σ uncertainty, which is 6.1 × 10−12 (cm3/molecule)/s from Sander et al.10 Given that the standard k1a rate constant alone results in more rapid production of HNO3 than we observe, it is straightforward to conclude that any additional production from the reaction of HO2 with NO to produce HNO3 (R2b) must be accompanied by further slowing of the k1a rate. In the air mass we are evaluating, HO2 was constant to within 25% at 6.7 × 107 molecules/cm3. If the k1a rate were zero, then the measurements would give the k2b of 1.9(±1.0) × 10−13 (cm3/ molecule)/s. If this were the case, we calculate a branching ratio of 1.7% for HNO3 production in the HO2 and NO reaction. On the contrary, if we assume the production by OH and NO2 is 50% of the recommended value (∼4.4 × 10−12 (cm3/ molecule)/s), then we find a branching ratio no larger than 0.8% for reaction R2b. This is similar to the laboratory value.11−13 For the reaction R2b branching ratios in the range of 0 to 0.7%, we find k1a ranges from 6.2(±2.9) − 4.7(±2.6) × 10−12 (cm3/molecule)/s, respectively. 4.2. Speciated Peroxy Nitrate Production. The observed production rates of PAN, PPN, CH3O2NO2, and HO2NO2 versus NO2 are shown in Figure 7. The calculations to produce Figure 7 are similar to the HNO3(g+p) production and described in section 4.2. Unlike HNO3, we do not have observations of the respective radical species that produce the peroxy nitrates, except for HO2NO2. We use the observations (Table 1) to calculate the production rates of the respective

Figure 5. Observed HNO3(g+p) production rate [(molecules/cm3)/s] versus NO2 (molecules/cm3). The solid red line is the slope (6.2 × 10−5 s−1), and the dashed-dot line is the 2σ uncertainty (±2.0 × 10−5 s−1). E


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Figure 7. Observed (a) PAN, (b) PPN, (c) CH3O2NO2, and (d) HO2NO2 production rate [(molecules/cm3)/s] versus NO2 (molecules/cm3). The solid red line is the slope, and the dashed-dot red lines are the 2σ uncertainty. The slopes are (a) 4.5(±1.0) × 10−5 s−1, (b) 6.8(±1.2) × 10−6 s−1, (c) 8.4(±4.8) × 10−5 s−1, and (d) 3.4(±1.8) × 10−5 s−1. For (c), 3 min averages are used to reduce the noise of the measurements.

Table 2. Calculated and Recommended Rate Constants (±2σ Uncertainty) for Speciated Peroxy Nitrates at T = 225 K and P = 230 hPaa species PAN PPN CH3O2NO2 HO2NO2

calculated rate constant 1.3(±0.8) 7.3(±3.4) 7.2(±5.7) 5.1(±3.1)

× × × ×

Sander et al. recommendationsb

recent laboratory results

−11 1.3+0.4 −0.3 × 10 +10.0 7.9−4.4 × 10−12 −12 5.7+3.9 −2.3 × 10 −12 1.2+0.4 × 10 −0.3

N/A N/A 6.4(±0.5) × 10−12 c −13 d 6.7+2.2 −1.7 × 10

−11

10 10−12 10−12 10−13

a

Rate constants used to calculate the peroxy radical species are found in Tables S1−S3. bReference 10. cReference 18. Rate is measured at 223 K and 267 hPa. dReference 14.

Figure 8. (a) Observed O3 versus ΣANs(g+p) during the flight. The slope (2σ uncertainty) is 25(±6). (b) Calculated cumulative production of alkyl and multifunctional nitrates (area plot in pptv) and the 15 min average (±1σ of the mean) measured production (red error bars in pptv).

the peroxy nitrate lifetimes are calculated to be longer than the length of the experiment (3.5, 13, and 464 h for HO2NO2, CH3O2NO2, and PAN/PPN, respectively). Also, the HO2NO2 produciton rate is calculated to be a factor of 1.5−3 higher than the loss rate; therefore, the losses of the peroxy nitrates are not

radical species using the rate constants found in Tables S1−S3. For the case of PPN, we use the relationship observed by Singh et al.48 between acetaldehyde and propanal to estimate propanal, which was not observed. Then, we use the calculated radical production rate and observed slopes from Figure 7 to converge to a radical concentration and NO2 rate constant. All F


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short lifetimes suggest that the ΣANs(g+p) production rate will not be sustained downwind of lightning NOx injection into the upper troposphere and only impacts the NOx lifetime in the near-field of the NOx injection. Observations of ΣANs(g+p) in the upper troposphere were typically ∼90 pptv, which is a factor of 2−4 lower than the ΣANs(g+p) mixing ratios in the outflow of convection, confirming that ΣANs(g+p) production is primarily important in the near-field (Figure 3c). The multifunctional organic nitrates produced after radical isomerization or from biogenic hydrocarbons at the low temperatures of the upper troposphere are expected to have low vapor pressures and condense onto aerosol particles. During the flight, an apparent production rate of 2.2 × 10−3 μg/m3 (at ambient T and P), or 3 pptv, ΣANs(p) was observed (Figure 9). We use the method described by Fry et al.37 to differentiate between NO3−(p) and ΣANs(p). This accounts for approximately 7% of the ΣANs(g+p) observed during the flight.

important to the assessment of the speciated peroxy nitrate production rate constants. The calculated rate constants for the speciated peroxy nitrates are compared with the recommended rate constants in Table 2. The uncertainty in the rate constants, ranging from ∼50 to ∼75%, is defined by the uncertainty in the slopes and the spread in the radical concentrations (±25% for the interquartiles) or the uncertainty in the HO2 concentration (1σ ≈ 16%). We find agreement (less than 30% difference) between the recommended10 and calculated rate constants for the peroxy nitrates except for HO2NO2. Our calculated rate constants support the current recommendations10 for the production rates of PAN, PPN, and CH3O2NO2 for the conditions of the upper troposphere and the findings of Bacak et al.,14 that the HO2NO2 production is slower than the recommendation by ∼50%. The lower HO2NO2 is also consistent with the conclusions of Kim et al.,31 that the current recommendations10 produced more HO2NO2 than was observed in the upper troposphere. To our knowledge, this is the first time that the PAN and PPN production rates have been reported for the conditions representative of the upper troposphere. 4.3. Alkyl and Multifunctional Nitrate Production. To evaluate the production of ΣANs(g+p), we compare ΣANs(g+p) and O3 mixing ratios (Figure 8a). As described by Perring et al.,49 the slope of a plot of O3 versus ΣANs(g+p) is approximately equal to 2/α, where α is the ratio of k3a/(k3a + k3b), as seen in eqs 1−3. This assumes all the hydrocarbons oxidized produce two O3 molecules (γ = 2), meaning that 2 peroxy radicals are rapidly produced following the photooxidation of the hydrocarbon.49 P(O3) =

∑ γi(1 − αi)k OH + RH [OH][RHi] i

i

P(ΣANs(g + p)) =

∑ αik OH + RH [OH][RHi] i

i

P(O3) =

(1)

Figure 9. Observed (black at ambient T and P) and calculated (blue) cumulative production of the ΣANs(p). The observations are 15 min averages (±1σ of the spread) and the pvap is reduced by a factor of 3.

(2)

γ ̅(1 − α̅ ) 2 P(ΣANs(g + p)) ≈ P(∑ ANs(g + p)) α̅ α̅

We calculate the vapor pressures of the individual ΣANs(g) using the SIMPOL.1 model52 and the contribution to the aerosol phase using the equilibrium absorptive partitioning formalism53 (eqs 4 and 5 and Table S5). The largest contribution to the ΣANs(p) are the monoterpene hydroxy nitrates, isoprene dihydroxy dinitrates, and hydroxy nitrates formed from the isomerization of n-hexane, methylhexanes, and n-heptane (Table S5). Explanation of the variables is found in the Supporting Information.

(3)

From the O3 versus ΣANs(g+p) correlation, we calculate an effective branching ratio, α̅ , (1σ) of 8(±2)%. For comparison, we calculate the ΣANs(g+p) production rate directly using the temperature and pressure dependent parametrization of ΣANs(g+p) formation from Carter and Atkinson50 and the observations of the various hydrocarbons (Tables 1 and S4 and description of calculation in Supporting Information). Lee et al.51 showed that at 273 K, ΣANs(g+p) production is 50% more efficient than at 300 K and even larger increases are calculated here for T = 225 K (250% more efficient). The direct calculation gives an average (1σ) α̅ of 7(±4)%. The uncertainty in α̅ includes uncertainty in the rate constants (∼15%), the hydrocarbon concentrations (∼10%), the OH concentration (∼16%), and the temperature and pressure dependent α value (∼30%). The calculated increase of ΣANs over the quasi-Lagrangian portion of the flight is (46(±18) pptv) (Figure 8b). In this air mass, the hydrocarbons that produce the largest fraction of the ΣANs(g+p) (∼70% or 34 pptv) are a group that have chemical lifetimes of less than 1 h (MVK, MACR, isoprene, and monoterpene) and a group with longer lifetimes but still less than 6 h (greater than C5 hydrocarbons). These

Kp =

R ·T ·fom MWom · 106 ·ζ ·pvap

(4)

m ANi,p = K p, i·mom ·m ANi,g

(5)

The calculated ΣANs(p) at the end of the experiment is 5.8 × 10−4 μg/m3, which is a factor 4 lower than observations. Prior studies have concluded that the SIMPOL.1 model can overestimate the vapor pressures of multifunctional nitrates by up to 2 orders of magnitude, which is compounded by the need to estimate them at unusually low temperatures (225 K).54−56 Given the prior discrepancy, a factor of 4 difference is not surprising. If the estimated vapor pressures are reduced by a factor of 3 (well within the uncertainties), then good modelmeasurement agreement is observed (Figure 9). G


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5. ATMOSPHERIC IMPLICATIONS Our results indicate that both the HNO3 and HO2NO2 production rate are slower in the upper troposphere than in current models, consistent with recent reports.19,31,57 Production of ΣANs(g+p) depends on temperature and, as a result, is more rapid than in some current models. Production of CH3O2NO2, PAN, and PPN are observed to occur at the rates currently recommended, although many models do not include CH3O2NO2. Incorporating all of these changes and using the observations during the DC3 experiment to constrain a steady state model, we find the NOx lifetime increases by 20% (Figure 10), leading up to as much as ∼10 h increases in the NOx lifetime, and consequently, to higher O3 production.

Figure 10. Comparison of the NOx lifetime (NOx lifetime = NOx/ Loss of NOx) in the upper troposphere (T ≤ 230 K) from a steady state model during the entire DC3 campaign. The base case uses the recommended rate constants from Sander et al.,10 and the updated chemistry case uses the rate constants from Henderson et al.47 and Bacak et al.14 The slope (±1σ) is 1.20(±0.01).

Some recent model studies have added the reaction R2b yield without adjustment of the HNO3 production rate.58−60 Not surprisingly, these models result in too little NOx and too much HNO3 compared to observations. One study19 decreased the HNO3 production rate on the basis of the recommendations from Henderson et al.,47 who analyzed upper tropospheric NOx observations from the Intercontinental Chemical Transport Experiment−North America, and the authors did not include reaction R2b. The slower HNO3 production rate decreased the HNO3 concentrations by ∼5−20 pptv and increased the NOx concentrations by a similar amount. Bacak et al.14 calculated the impact of the slower HO2NO2 production rate on HO2NO2 and NOx. They report a decrease in HO2NO2 of 30−40% and an increase in NOx of 5−20%. To investigate the impacts of the slower HNO3 and HO2NO2 production compared to current recommendations, we use GEOS-Chem v9-02b (details in the Supporting Information) to model the year 2012 (Figure 11). The largest NOx increases (>15 pptv for the entire year) occur over regions (central Africa, southeast United States, and northern India) of high lightning and deep convection.61 The combined effects of slower HNO3 and slower HO2NO2 production leads to larger changes in NOx and O3 concentrations than calculated with only a slower HNO 3 rate. 19 The increases in NO x concentrations are associated with decreases in HNO3 and HO2NO2 concentrations (up to 20 and 15 pptv, respectively) and increases in OH, PAN, CH3O2NO2, and O3 concentrations

Figure 11. Absolute annual mean change in (a) NOx, (b) HO2, (c) OH, (d) O3, (e) HNO3, (f) PAN, (g) HO2NO2, and (h) CH3O2NO2 in the upper troposphere (200−400 hPa). The mean change is calculated as (updated chemistry case−base case). Values above the tropopause are removed. Note the different scales for the color bars.

(up to 0.04, 15, and 5 pptv and 5 ppbv, respectively). HO2 concentrations change little due to the competing effects of decreased HO2NO2 production and increased NO concentrations. The increases in both PAN and CH 3 O 2 NO 2 concentrations are associated with O3 increases. The increased production of these less stable NOz species allows transport of NOx and leads to O3 production downwind of the NOx source.62

6. CONCLUSIONS We have directly measured the production rates of speciated peroxy nitrates, alkyl and multifuctional nitrates, and HNO3 in the upper troposphere. Using these observations to infer rate constants for the reactions at the temperature and pressure of the upper troposphere, we find that the rate of OH and NO2 (k1a) to produce HNO3 is slower than currently recommended, making peroxy nitrate production as important as HNO3 production as a sink of NOx in the near field of lightning NOx injection. The observations indicate that any HNO3 production from the reaction of HO2 with NO is small, H


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The Journal of Physical Chemistry A which suggests the combined effects of k1a and k2b should not exceed the estimate for k1a when k2b is assumed to be zero. We also calculate that the HO2NO2 production rate is ∼50% slower than currently recommended, supporting conclusions of Bacak et al.14 We show that in the near field of deep convection, the effective average branching ratio to forming ΣANs(g+p), α̅ , is high (∼8%) and dominated by short-lived hydrocarbons. Combining these ideas in a global chemical transport model, we demonstrate that decreases in the HNO3 and HO2NO2 production rates lead to increases in the calculated upper tropospheric NOx (10−14%), OH (10−13%), and O3 (5−6%) concentrations.

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edge funding form NASA (NNX12AC03G and NNX15AH33A). JD and ES acknowledge funding from NASA (NNX12AB80G). BAN would like to thank Joshua L. Laughner in his assistance in setting up and running the GEOS-Chem model. The authors also want to thank the ground and flight crews of the DC-8 and the DC3 science team. Finally, the authors want to thank Donald Blake for the use of the whole air sampler measurements, Glenn Diskin for the use of the CH4 observations, and the reviewers for their constructive and thoughtful comments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b07824. Description of the instrument intercomparisons for HO2NO2, peroxy nitrates, and HNO3 and of the NO2 photostationary steady state calculations, description of the calculation of alkyl and multifunctional nitrate production, description of variables in eqs 4 and 5, and description of the GEOS-Chem model. Intercomparisons of observations and of the intercomparison between photostationary steady state and NO2 observations (Figures S1 and S2). Full flight path of the DC-8 during the experiment (Figure S3). Time series of speciated peroxy nitrate observations (Figure S4). First-order decay of toluene and ethyne (Figure S5). Rate constants used to calculate peroxy and acyl peroxy radical concentrations (Tables S1−S3). Parent compounds, rate constant, initial concentrations, branching ratios, and classification of product for Figure 8b (Table S4). Parent compound, vapor pressure, percent contribution to aerosol particle, and percent contribution to calculate ΣANs(p) (Table S5). (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*R. C. Cohen. Phone: (510)-642-2735. E-mail: rccohen@ berkeley.edu. Present Address %

Atmospheric Science Department, Colorado State University, Fort Collins, Colorado 80523, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS BAN was supported by the National Science Foundation Graduate Research Fellowship under grant no. DGE 1106400. BAN, CG, PWJ, and RCC acknowledge funding support from NASA (NNX12AB79G). PTR-MS measurements aboard the NASA DC-8 were supported by the Austrian Federal Ministry for Transport, Innovation and Technology (bmvit) through the Austrian Space Applications Programme (ASAP) of the Austrian Research Promotion Agency (FFG). TM was supported by an appointment to the NASA Postdoctoral Program at the Langley Research Center, administered by Oak Ridge Associated Universities through a contract with NASA. JDC and POW acknowledge funding support from NASA (NNX12AC06G and NNX14AP46G). PCJ and JLJ acknowlI


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